Taking a plunge at last

Discussion in 'Sailboats' started by Lurch723, Aug 22, 2012.

  1. Lurch723
    Joined: Aug 2012
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    Location: Devon UK

    Lurch723 Junior Member

    Since I was a kid I've been designing boats and over the years many of my ideas have since been realised by other people while I've been wasting my life away in publishing and in latter years aircraft engineering. However I have finally decided to bite the bullet, stick my head above the trench and build something I've designed.

    I'm not interested in things with lumps of lead underneath and don't want to produce yet another short wide planing dinghy with racks and trapezes. Instead I want to design that's graceful, packs great upwind speed in waves and lifts and planes downwind like a skiff. The other influencing factors are that it can be sailed by those of us who aren't quite at their physical peak if you get my way of thinking (over the hill).

    I am thinking along the lines of a 20ft long 18" wide monohull that carries a crew of two on wings 8-10ft wide with outriggers. These outriggers would be only sufficient to work as dampeners for the crew to buy them time to get their weight in in lulls and shifts, maybe enough volume to float 2 people. Sailplan: not decided (yet to influenced) looking at fully automatic 49er style rig or maybe go the other way and look at a cat style rig with a high aspect ratio main and jib but a more 49er style kite for deeper offwind work.

    What I am trying to do is design a widened planing cat hull that uses floats and carries a generous sailplan. I really like the idea of a double ender AKA Int' canoe, but am struggling to find a good enough reason why I should. It adds waterline length and offers potentially a better non planing performance but surely it's just form drag when moving at planing speeds?

    Construction - as usual foam core/carbon layup. 10mm PVC core with bi-directional both sides. Was also looking at marine ply inner skin, foam core then carbon outer but unsure how reliable this would be? Main rig compression loads taken by an alloy welded subframe that forms a part of the front wing spar arrangement and is bolted into the boat at 4 separate locations (for/aft and port & stbd). No lifting foils at this stage as I would be concerned with launching and recovery too much.
     
  2. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    An interesting and very ambitious set of design goals!

    Based on what I've heard from folks who sail International Canoes they are not an easy boat to sail.

    I'm confused by two of your statements:

    "I am thinking along the lines of a 20ft long 18" wide monohull that carries a crew of two on wings 8-10ft wide with outriggers."

    "What I am trying to do is design a widened planing cat hull that uses floats and carries a generous sailplan."

    The first statement sounds like a trimaran with minimal volume amas. A 20 ft long, 18" wide hull (L/B - 11.3) is not generally considered to be a planing hull.

    Attached are several photos of boats with tubular subframes for distributing the rig loads on the hull. The boats are a local class from the Shetland Islands.
     

    Attached Files:

  3. Lurch723
    Joined: Aug 2012
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    Lurch723 Junior Member

    Thanks for posting the space frame images, yes something similar to that. My apologies for creating confusion, I do indeed want to produce a slimmed down planing platform and wanted to use the length to get the lift. Not sure if this can work in practice? what is the minimum planing ratio required? I was hoping to use the length of the mid and aft section volumes to promote planing.
     
  4. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    Mono tri

    Heres a picture of Mirabaud-longer than you want but the hull seems similar to what you want. Also the main hull of the Weta trimaran is a planing hull. You might consider a scaled up Weta keeping the "amas" the same length so that they are basically just buoyancy pods. The more toward a trimaran you go the easier the boat will be to sail. Thomas Jundt/Mirabaud tells me that foils or no foils the boat is a handfull in roll though very stable in pitch. Have you considered using hydrofoils? Have you seen Bethwaites "HSP"?
    Picture below-basically a narrow plaing monohull with planing buoyancy pods:

    Keeping in mind that the Weta uses a planing main hull and a nominally displacement ama here is a thread with the technical specs of the boat including main hull Length/Beam@wl ratio and other important stats: http://www.boatdesign.net/forums/multihulls/weta-normal-turbo-proposal-36228.html

    click- Pictures, L to R-1) Mirabaud, 2) Weta, 3) Bethwaites HSP.
     

    Attached Files:

  5. Manie B
    Joined: Sep 2006
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    Location: Cape Town South Africa

    Manie B Senior Member

    well

    what you want and what you are capable of handling may be an issue
     
    1 person likes this.
  6. CutOnce

    CutOnce Previous Member

    Truer words have rarely been written here.

    Many design briefs posted here are actually fantasy-based wish lists, ignoring the realities of time commitment, money, building/sailing abilities and making the hard choices and compromises required to get a design on the water.

    I wish the original poster every success in his venture, but cherry picking performance highlights from several designs isn't acknowledging the compromises those designs made. ICs are deadly fast upwind - but they pay a stability price and a downwind speed price. Skiffs are only stable at high speeds, and best downwind - conceding upwind speed, ability to handle chop and requiring instantaneous prediction of where crews need to be on the wire to avoid swimming constantly. No ultra high performance boats tolerate any excess weight happily.

    I wish there were a magic formula to design a boat incorporating the best of everything, avoiding compromises and lowering the bar for the sailor, but it just isn't possible as far as I can see. Choosing what you are willing to give up is equally important to identifying what you want.

    I've been dealing with some hard realities this past year, with early onset arthritis and gout cutting my trapeze skiff sailing out of possible activities. I'm just not fast enough any more and the pain is too high to feel safe sailing a high performance boat alone. Especially one that regularly achieves it's goal of being upside down. Less than one kilometre downstream from my club's docks are shallow rapids, so slow capsize recovery sailing alone isn't an option.

    I'm now considering adding amas to my boat as I just don't want to give it up yet. I'd bought plans for an i550, but just can't concede that lead poisoning is my only future. I'm also considering one of Richard Woods cats - at least they don't need lead.

    Fight the good fight Lurch723, but remember to design your boat for where you will be at least two years from now physically (acknowledging build time). I wish I had done that when I built my single hand skiff - I'd be spending a lot more time on the water - although my boat would be slower.

    --
    CutOnce
     
  7. messabout
    Joined: Jan 2006
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    messabout Senior Member

    Marvelous reply Cutonce. You are expressing the wisdom of the ages.(pun intended and apologized for). A tip of the hat to Manie too. How come so many of you guys are so smart?

    I wish to hell I had the physical capacity to sail the IC or the A-cat that I had in the days before. Either of those were easy at the time. In fact I reckon the IC was about the easiest go fast boat that ever used me. Hiking planks were easier and faster than traps.. Ahhh Nostalgia.
     
  8. Doug Lord
    Joined: May 2009
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    Location: Cocoa, Florida

    Doug Lord Flight Ready

    Banshee Ambulance-Cherub equipped with amas

    From a thread in the Multihulls section:

    Pictures-Banshee Ambulance sketch and the Cherub it is based on sailing:

    click-
     

    Attached Files:


  9. Lurch723
    Joined: Aug 2012
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    Location: Devon UK

    Lurch723 Junior Member

    Some good reading there Doug, I am busy processing all this info and it may take a while to come out the other end of the sausage making machine! I think a stretched fast Weta is probably something approaching what I am after. However I want to look at it from a more race orientated angle, a purer Turbo Weta. Very encouraging though, the Mirabaud is way too extreme for me, it looks incredibly serious and sophisticated!

    Cutonce, you are quite right, what is the point of having a design with power and speed potential with no way of controling it - makes for a slow boat indeed - shiny side up. Don't confuse my reference to an IC as cherry picking Cutonce, I am simply illustrating a wish to explore the positives of creating a low drag for waterline length hull shape by allowing fluid to behave in a less turbulent way after max beam. I am unaware of how efficient this is at planing speed, hence my hesitation on introducing it.

    I think I need to go sail a Weta before I progress any design.
     
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